Employees who can exercise at work are more productive, happy, efficient and calm, a study shows.
Two hundred people took part in the Bristol University experiment to test the impact of workplace keep fit facilities like gyms or classes.
Exercise re-energised staff, improved their concentration and problem solving and made them feel calmer.
Experts said the findings should encourage more businesses to provide facilities for staff to exercise.
Indeed, almost half of the "Sunday Times 100 Best Companies to Work For 2006" offered gym access to employees, say the latest study authors.
However, their work in the Journal of Workplace Health Management, did find employees struggled to fit exercise around their work and felt guilty about being away from their desks.
They also felt they might be criticised by colleagues for taking time out from their desk jobs.
The volunteers were asked to fill out short questionnaires about their mood and work productivity on two working days of their choice - one set for a day they decided to exercise and a second set for another day.
Lead author Jo Coulson, from Bristol University, said: "It's generally well-known now that there are many physical and mental health benefits that can be gained from regular exercise.
"If people try to fit an active break into their working day, they might also experience the added bonus of their whole day feeling much more productive. And that always feels good in our busy lives.
"The study also begs the question whether employers can afford not to be encouraging active breaks.
"The suggestion is that employers who are ahead of the game in offering proper onsite facilities actually get less from their employees on days that they don't exercise."
A spokeswoman for the British Heart Foundation (BHF) said healthy workplaces could inspire us to become a fitter, trimmer nation.
"As we spend 60% of our day in the workplace, the BHF is encouraging employers to make them healthier places for their staff through our 'Think Fit' programme."